feat(offline): play downloaded video, and drain the offline sync queue (0.4.6)

Bundles this session's work plus the concurrent search/offline/player changes.
Every gate passes on the combined tree: 885 frontend tests, 610 Rust tests,
clippy clean, boundary clean, trace coverage 86%.

Offline video playback — four separate defects, each of which alone stopped it:

  DR-133  A completed download's file_path is already absolute (the worker
          rewrites it on completion), but the player rooted it a second time and
          handed the webview /data/user/0/app//data/user/0/app/videos/x.mp4.
  DR-134  The asset protocol was never enabled: no protocol-asset feature and no
          assetProtocol config, so convertFileSrc produced URLs nothing answered.
          Also silently defeated the cached-thumbnail path, which fails soft to
          the server copy and hid it whenever the server was reachable.
  DR-137  Tauri's asset protocol answers a range-less request by reading the
          whole file into memory, and only advertises Accept-Ranges from inside
          its range branch, so the first request never learns ranges exist.
          Chromium gave up with PIPELINE_ERROR_READ after ~31s. Local media is
          now served by a loopback HTTP server: bounded 4 MiB chunks streamed
          from the file handle, every response length-delimited, and a range-less
          request answered with one chunk rather than the file. Confined by a
          per-session token and to the app data directory, because loopback is
          shared between apps on Android.
  DR-138  Release builds set usesCleartextTraffic=false, so Android rejected the
          request to that server before any I/O. A network-security-config
          exempts 127.0.0.1 only; a remote server must still be HTTPS.

Downloads:

  DR-135  download_item never records media_type and the reconnect resolver read
          that NULL as 'audio', so a movie queued from a media card had its URL
          resolved by get_audio_stream_url and completed as an audio-only
          transcode. The item's own type now decides.
  DR-136  Rows already downloaded that way are requeued on reconnect, since
          prevention alone leaves them reading "downloaded" and still unplayable.

Known limitation: a download taken at `original` quality is a byte copy of the
source, so it can be any container. One such file is an AVI holding XVID, which
the webview cannot play in any case — the media server serves it correctly and
Chromium refuses it. That needs either a transcoded download preset or the
native ExoPlayer surface work, and is not addressed here.

Also fixes two ID collisions between concurrent work: DR-143 defined twice
(search vs offline gate) and UT-131 defined twice (Episode Focus hero vs channel
cap). The search requirement is now DR-147 and the channel-cap test UT-141, with
their code references and matrix rows updated.

src/lib/api/bindings.ts

@@ -873,6 +873,22 @@ async markDownloadCompleted(downloadId: number, bytesDownloaded: number, filePat
 async markDownloadFailed(downloadId: number, errorMessage: string) : Promise<null> {
     return await TAURI_INVOKE("mark_download_failed", { downloadId, errorMessage });
 },
+/**
+ * A playable URL for a downloaded file on disk.
+ * 
+ * Local media is served over a loopback HTTP server rather than handed to the
+ * webview as a `file://`/asset URL, because the asset protocol cannot stream a
+ * large file — it answers a range-less request with the whole thing, which
+ * Chromium abandons. See `media_server` for why real HTTP is used.
+ * 
+ * The returned URL carries the server's per-session token, so it is only valid
+ * for this run of the app and must not be persisted.
+ * 
+ * TRACES: UR-071 | DR-137
+ */
+async mediaLocalUrl(path: string) : Promise<string> {
+    return await TAURI_INVOKE("media_local_url", { path });
+},
 /**
  * Start downloading a file immediately
  * This command actually downloads the file using the worker
@@ -1130,6 +1146,14 @@ async syncMarkFailed(id: number, error: string) : Promise<null> {
 async syncGetPendingCount(userId: string) : Promise<number> {
     return await TAURI_INVOKE("sync_get_pending_count", { userId });
 },
+/**
+ * Push the queue now, on the user's say-so, instead of waiting for a reconnect.
+ * 
+ * TRACES: UR-025 | DR-132
+ */
+async syncProcessPending() : Promise<DrainReport> {
+    return await TAURI_INVOKE("sync_process_pending");
+},
 /**
  * Delete completed sync operations older than specified days
  */
@@ -1879,6 +1903,26 @@ export type DownloadVideoRequest = { itemId: string; userId: string; filePath: s
  * Enhanced response with pre-computed stats
  */
 export type DownloadsResponse = { downloads: DownloadInfo[]; stats: DownloadStats }
+/**
+ * What a drain did, for logging and for the frontend's "Sync now" button.
+ */
+export type DrainReport = { 
+/**
+ * Rows that reached the server and are now `completed`.
+ */
+pushed: number; 
+/**
+ * Rows that failed and will be retried on the next reconnect.
+ */
+deferred: number; 
+/**
+ * Rows that exhausted `MAX_SYNC_ATTEMPTS` and were given up on.
+ */
+abandoned: number; 
+/**
+ * Rows still waiting afterwards (what the badge counts).
+ */
+remaining: number }
 /**
  * Built-in equalizer presets. A preset *is* a gain curve defined by the band
  * layout above (a domain concept), not a mere label — the curve numbers live
@@ -2750,7 +2794,15 @@ mime_type: string }
 /**
  * Sync queue item returned to frontend
  */
-export type SyncQueueItem = { id: number; userId: string; operation: string; itemId: string | null; payload: string | null; status: string; retryCount: number; createdAt: string | null; errorMessage: string | null }
+export type SyncQueueItem = { id: number; userId: string; operation: string; itemId: string | null; payload: string | null; status: string; retryCount: number; createdAt: string | null; errorMessage: string | null; 
+/**
+ * Cached title of the item the operation is about, when the catalog knows
+ * it. Resolved here rather than by a per-row frontend fetch — the queue
+ * list is otherwise a wall of opaque ids.
+ * 
+ * TRACES: UR-025 | DR-132
+ */
+itemName: string | null }
 /**
  * Statistics about the thumbnail cache
  */